In short, yes. I've been using a 901 for the past 10 years. Currently on my second one. I've heard the 501 Mk II at various friends' places.

I would say that the 501 tends to sound "warmer". This could be that the 501 is not as extended on both ends as the 901, or that the 501 accentuates the midrange more than the 901. I feel that the 901 is more analytical, meaning it has more detail, and has a more balanced sound across the spectrum.

I had a 901, currently have the Shelter 501-II. The 901 definitely had more going on in the highs and lows, but less than the 501 in the mid-range. I love the 501, and only liked the 901, which I found more impressive but less immersive.